Dry methods for separation of mineral contaminants from coal were used in the 1800's and peaked around 1965 when wet methods which are more effective in mineral separation were developed. With the growth of concern for the massive volumes of wet refuse to be managed, however, interest is again growing in the development of dry methods for ore dressing. Additionally, in arid regions of the world where water is not available for wet processing, dry methods have always been of interest.
Air jigs are now being considered again for dry cleaning of raw coal. (J. K. Alderman and R. J. Snoby, “Improving Power Plant Performance and Reducing Emissions through the use of Pneumatic Dry Cleaning of Low Rank Coal,” Preprint 01-120, 2001 SME Meeting, Feb. 26-28, Denver, Colo., incorporated by reference herein). However, air jigs are ineffective in cleaning coal particles larger than 50 mm and smaller than 0.6 mm diameter (R. P. Killmeyer, Jr. and A. W. Deurbrouck, “Performance Characteristics of Coal-Washing Equipment: Air Tables,” Report of Investigations PMTC679, April, 1979, incorporated by reference herein). It is not unusual for a significant portion of the raw coal to have sizes smaller than ¼ inch. Further, air jigs are generally limited to “black-and-white” separations at 1.6 specific gravity or higher. Separations at specific gravities much below 1.6 are simply not feasible. Pneumatic cleaning of coal particles which have a wide range of particle sizes can be complex and expensive. One approach is to screen the coal into coarse and fine sizes and to treat each separately. This is undesired because of the poor performance at fine sizes and because it increases costs. Alternatively, small particles can be lost to the process by discarding the fine particles screened out before jigging or by discarding the fine coal blown through the jig which is collected in a bag house or other method used to keep dust to a minimum. Either of these approaches represents a severe loss in heating value. Alternatively, the fine particles can be collected with the coarse product but this will raise the ash and sulfur levels in the cleaned coal. Indeed, the concentrations of mineral contaminants, especially pyritic sulfur, tends to be higher in the fine fraction than in the coarser components of most coals. All together, the ineffectiveness of the air cleaning devices in treating fine coal has limited the application of this technology.
Air jigs have specific advantages associated with the use of pulsating air rather than water. They have specific disadvantages also in that separation of coal particles larger than 45-50 mm is virtually impossible. Further, processing of unsized feeds such as 50 mm topsize results in excessive misplaced material. Additionally, the air jig like many other dry processes, has a practical upper surface moisture limit of about 6%. Lastly, dust control is a necessity.